It is necessary to remove field heat and chill produce to preserve the produce during shipment to market. Some methods of cooling produce appear in U.S. Pat. No. 3,844,132, Produce Cooler and Method of Cooling Product, Issued on Oct. 29, 1974, and in U.S. Pat. No. 4,576,014, issued Mar. 18, 1986, and entitled Improved Vacuum Cooler With Improved Venting. These patents describe apparatus comprising a vacuum chamber for receiving the produce. A vacuum pump reduces the pressure within the chamber to sub-atmospheric level, causing evaporation of moisture from the produce. This evaporation removes heat from the produce, thereby reducing its temperature. Water vapor formed by such vaporization condenses on cooling coils positioned over the produce.
Additionally, a recirculation system picks up water from a reservoir at the bottom of the vacuum chamber and sprays it over the produce for a further cooling effect.
Such chambers include a venting system to permit outside air to flow into the vacuum chamber, and to raise the pressure inside the chamber to atmospheric pressure before the chamber is opened.
One result of a rapid pressure increase within the vacuum chamber is vacuum infiltration of moisture into the produce. Softer, more porous produce such as lettuce, cabbage and spinach are especially vulnerable to such infiltration. This phenomenon occurs when water lying on the surface of the produce during rapid pressurization is forced into the produce because the internal pressure of the produce rises more slowly than its external pressure. While no permanent harm is done to the produce, the water saturated area remains until evaporation of the water occurs. During this period, water-infiltrated areas appear bruised or discolored, and are unappealing to buyers.
In U.S. Pat. No. 4,576,014, vacuum venting takes place through a separate pipe extending into the vacuum chamber beneath the surface of the water in the sump area. When the vacuum is released beneath the water surface, the water can erupt, coating the produce with unwanted water from the sump. This unwanted water can accelerate rot and spoilage during storage and shipment.
It is an object of this invention to provide a vacuum cooling apparatus which minimizes both vacuum infiltration of the produce, and unwanted water applications to the produce.